The spatial pattern of slip rates along the Haiyuan fault zone may reveal deformation process along the northeastern margin of the Tibetan Plateau. By applying the 3D_Fault_Offsets code, we reevaluated the displacements of offset markers displaced by the Laohushan fault, the central segment of the Haiyuan fault zone. We introduced the Bayesian Age Model approach to provide optimum age control for the preserved offset markers. Multiple offsets and ages constrain the late Pleistocene slip rate to 4.4–4.8 mm/yr. The spatial pattern of strike‐slip rates of the Haiyuan fault zone indicates that the Madongshan and Liupanshan could be the main uplifts that absorbed the left‐lateral motion along the Haiyuan fault zone, due to a restraining bend between the Haiyuan and Qishan‐Mazhao fault zones. Furthermore, uniform slip rates along the main segments and slip rates decreasing toward fault tips are widely observed along some intensively studied strike‐slip faults, such as the Altyn Tagh, Kunlun, Death Valley‐Fish Lake Valley, Denali, San Andreas and Alpine faults. The results suggest that the constancy or similarity of the slip rates along the strike is closely related to block rotation and internal deformation, and the slip rates of strike‐slip faults usually change at the fault tip area converting from horizontal to vertical motion. In addition, strike‐slip fault can also take slip off of the main fault and distribute it between multiple active parallel strands.
The northwest striking Qishan‐Mazhao fault (QMF) accommodates complex deformation in the Tibet‐Ordos transition zone. We studied the geologic and geomorphic expression of the QMF using interpretations of high‐resolution satellite images and structure‐from‐motion models combined with detailed field investigations. Displaced loess tablelands, stream channels, and fluvial terraces show that the QMF is predominately a left‐lateral strike‐slip fault with a minor normal component. The magnetic susceptibility and optically stimulated luminescence ages of offset fluvial terraces yield left‐lateral slip rates ranging from 0.5 to 1.0 mm/year. Regionally, the QMF and the Haiyuan fault (HYF) form a large right step, in which the Liupanshan Mountains are located. The QMF shares a similar orientation and sense of motion to the HYF, suggesting that the left‐lateral slip of the HYF is not completely absorbed as crustal shortening across the Liupanshan Mountains but is partially transferred to slip along the QMF.
6 May 2021 in Journal of Geophysical Research: Solid Earth
The northeastern Tibetan Plateau and its surroundings are a tectonically complex region resulting from the continuing collision of India with Eurasia. We studied the tectonic deformation of this region using a GPS block model, which was produced using the most recently published, dense, precise, and complete GPS velocities available combined with a detailed fault geometry. The comparison between our preferred model with numerous faults and a model with fewer faults demonstrates the need for inclusion of lesser active faults affords a closer examination of strain partitioning. The predicted senses of motion and slip rates of active faults in our preferred model are consistent with the geological estimates, supporting the assumption that the decadal GPS slip rates are representative of millennial‐scale geologic slip rates. The observed fault kinematics in the northeastern Tibetan Plateau and its surroundings can be interpreted as the consequence of relative motions between different blocks and block rotations.
The quantification of the geometry and kinematics of the boundary faults of an active orogenic belt is key to exploring its deformation pattern. The Kashihe fault (KSHF), which bounds the northern margin of the Ili Basin and is more than 400 km long, is a WNW‐striking fault in the northern Tian Shan. Although the KSHF displays active faulting and is the seismogenic fault of the 1812Ms 8 Nilka earthquake, the quantitative slip rate and its role in regional strain accommodation have not been reported. Here, we quantify the late Quaternary activity of the KSHF based on high‐resolution remote sensing image interpretations and detailed field investigations. Six field observation sites along a ~270 km active fault in the Chinese Tian Shan indicate that the KSHF is characterized by dextral strike‐slip faulting and N‐S thrusting. Based on the surveying of offset geomorphic surfaces with an unmanned drone and the dating of late Quaternary sediments using optically stimulated luminescence (OSL) and radiocarbon methods, we estimated a late Quaternary right‐lateral strike‐slip rate across the fault of 2.39 ± 0.55 mm/yr and a shortening rate of 0.62 ± 0.30 mm/yr. Analysis of field investigations and fault section exposing indicated that a young paleoearthquake event occurred on the western segment of the KSHF. Active tectonics around the Ili Basin imply that the rigid Ili block extrudes westward along its boundary strike‐slip faults and rotates counterclockwise. We propose that this block rotation driven by dextral faulting accommodates regional N‐S convergence and sinistral shearing.
Spatial patterns in fault activity were examined along the Yanggao‐Tianzhen Fault (YTF) in the northern Shanxi Graben, North China. First, we reconstructed the distribution of fault slips and quantified slip rates at seven sites along the fault, based on satellite image interpretation, field investigation, topographic surveying, and optically stimulated luminescence dating. The T3terraces and their contemporaneous alluvial fans with a best‐estimate age of 31.7 ± 1.4 kyr were used as strain markers to estimate slip magnitudes. We determined vertical slip and dip‐slip rates of ∼0.10–0.31 mm/yr and ∼0.11–0.37 mm/yr, respectively, at the surface. Assuming fault dips of 40 ± 10° at seismogenic depths, we predicted extension rates of ∼0.17–0.55 mm/yr. This suggests that at least 10% and up to 28%–54% (95% confidence interval) of the total extension of the northern Shanxi Graben is inhomogeneously partitioned by different sections of the YTF. We also evaluated the along‐fault relative tectonic activity by analyzing of five widely used geomorphic indices: hypsometric integral, basin elongation ratio, asymmetric factor, valley‐floor‐width‐to‐height ratio, and mountain front sinuosity. The determined fault slip rates and geomorphic indices demonstrate that the most prominent fault activity center occurred in the middle of the fault, and that fault activity wanes toward the fault tips. Moreover, the fault activity is highly heterogeneous and fluctuates along the fault strike, which may be attributed to either fault segmentation and linkage or the possible strike‐slip component of motion along the fault. Further exploration of fault kinematics and paleoseismic history is warranted.